Pace discrimination of tachycardia using atrial-ventricular pacing

ABSTRACT

A cardiac medical device and associated method control delivery of dual chamber burst pacing pulses in response to detecting tachycardia. In one embodiment, a single chamber pacing pulse is delivered in response to detecting a tachycardia. Dual chamber pacing pulses are delivered subsequent to the single chamber pacing pulse. An intrinsic depolarization is sensed subsequent to delivering the dual chamber pacing pulses. The tachycardia episode is classified in response to the sensed intrinsic depolarization.

CROSS-REFERENCE TO RELATED APPLICATION

Cross-reference is hereby made to the commonly-assigned related U.S.Published Applications: U.S. Publication No. 2011/0077703 A1, entitled“PACE OF TACHYCARDIA USING ATRIAL-VENTRICULAR PACING”, to Brown et al.,and U.S. Publication No. 2011/0077704 A1, entitled “PACE OF TACHYCARDIAUSING ATRIAL-VENTRICULAR PACING”, to Brown et al., both filedconcurrently herewith and incorporated herein by reference in theirentireties.

TECHNICAL FIELD

The present disclosure relates generally to cardiac medical devices andto a device and method for monitoring the heart rhythm and deliveringatrial-ventricular pacing to the patient for use in discriminating heartrhythms.

BACKGROUND

A typical implantable cardioverter defibrillator (ICD) has thecapability of providing a variety of anti-tachycardia pacing (ATP)regimens. Normally, these regimens are applied according to apre-programmed sequence, and each regimen includes a predeterminednumber of pacing pulses. After the series of pacing pulses is delivered,the device checks to determine whether the series of pulses waseffective in terminating the detected tachycardia. Typically,termination is confirmed by a return to either a demand-paced rhythm ora sinus rhythm in which successive spontaneous depolarizations areseparated by at least a defined interval. If the tachycardia is notterminated, the ICD device may deliver a subsequent series of pacingpulses having modified pulse parameters, e.g. reduced inter-pulseintervals and/or an altered number of pulses. When ATP attempts fail toterminate the tachycardia, high-voltage cardioversion shocks may bedelivered. Since shocks can be painful to the patient and consumerelatively greater battery charge than pacing pulses, it is desirable toavoid the need to deliver shocks by successfully terminating thetachycardia using less aggressive pacing therapies.

The success of a tachycardia therapy depends in part on the accuracy ofthe tachycardia detection. In some cases, a tachycardia originating inthe atria, i.e. a supraventricular tachycardia (SVT), is difficult todistinguish from a tachycardia originating in the ventricles, i.e. aventricular tachycardia (VT). For example, both the atrial chambers andthe ventricular chambers may exhibit a similar tachycardia cycle length(P-P intervals and R-R intervals respectively) when an SVT is conductedto the ventricles or when a VT is conducted retrograde to the atria.Accordingly, methods are needed for accurately classifying a detectedtachycardia as VT or SVT to allow the most appropriate therapy to bedelivered by the ICD, with the highest likelihood of success and withoutunacceptably delaying attempts at terminating the tachycardia.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a cardiac medical device.

FIG. 2 is a functional block diagram of the cardiac medical device shownin FIG. 1 according to one embodiment.

FIG. 3 is a timeline of a VT episode falsely detected as SVT.

FIG. 4A is a timing diagram illustrating one method for delivering burstpacing in the atrial and ventricular regions of a patient's heart fordiscriminating SVT and VT.

FIG. 4B is a timing diagram showing a portion of the timeline of FIG. 4Ain greater detail.

FIG. 5 is a flow chart of a method for delivering dual chamber burstpacing for use in discriminating SVT and VT.

FIG. 6 is a flow chart of a method for discriminating VT and SVTaccording to an alternative embodiment.

FIG. 7 is a timeline showing a premature atrial pacing pulse deliveredat a paced VA interval (PVA) following the last ventricular sensedevent.

FIG. 8 is a timeline of a detected tachycardia having 1:1 correspondencebetween ventricular sensed events and atrial sensed events.

FIG. 9 is a flow chart of an alternative method for paced tachycardiadiscrimination.

FIG. 10 is a flow chart of a method for discriminating tachycardia usinganalysis of the post-paced event pattern.

FIG. 11 is a flow chart of an alternative method for dual chamber pacingdiscrimination of tachycardia.

DETAILED DESCRIPTION

In the following description, references are made to illustrativeembodiments. It is understood that other embodiments may be utilizedwithout departing from the scope of the invention. In some instances,for purposes of clarity, identical reference numbers may be used in thedrawings to identify similar elements. As used herein, the term “module”refers to an application specific integrated circuit (ASIC), anelectronic circuit, a processor (shared, dedicated, or group) and memorythat execute one or more software or firmware programs, a combinationallogic circuit, or other suitable components that provide the describedfunctionality.

FIG. 1 is a schematic representation of a cardiac medical device 10.Cardiac medical device 10 is embodied as an ICD in FIG. 1, however,methods described herein should not be interpreted as being limited toany particular implantable medical device or any particular cardiacmedical device. Instead, embodiments may include any cardiac medicaldevice so long as the device utilizes a plurality of electrodes or othersensors for monitoring the cardiac rhythm of a patient and fordelivering pacing pulses to the patient. The electrodes are capable ofsensing cardiac EGM or ECG signals, referred to herein collectively as“cardiac signals” in the upper atrial chambers and in the lowerventricular chambers.

In FIG. 1, heart 12 includes the right atrium (RA), left atrium (LA),right ventricle (RV), left ventricle (LV), and the coronary sinus (CS)extending from the opening in the right atrium to form the great cardiacvein. FIG. 1 depicts device 10 in relation to the heart 12. As shown,three transvenous leads 14, 16, and 18 connect the device 10 with theRA, the RV and the LV, respectively. Each lead includes at least oneelectrical conductor and pace/sense electrode. For example, leads 14, 16and 18 are respectively connected to pace/sense electrodes 20, 22, and24. In addition, a can electrode 26 can be formed as part of the outersurface of the housing of the device 10. The pace/sense electrodes 20,22, and 24 and can electrode 26 can be selectively employed to provide anumber of unipolar and bipolar pace/sense electrode combinations forpacing and sensing functions. The depicted positions in or about theright and left heart chambers are merely illustrative. Moreover, otherleads and pace/sense electrodes can be used instead of, or incombination with, any one or more of the depicted leads and electrodes.

Typically, in pacing systems of the type illustrated in FIG. 1, theelectrodes designated herein as “pace/sense” electrodes are used forboth pacing and sensing functions. In certain embodiments, theseelectrodes can be used exclusively as pace or sense electrodes inprogrammed or default combinations for sensing cardiac signals anddelivering pace pulses.

A coil electrode 34 is also shown as being coupled to a portion of RVlead 16. Coil electrodes can additionally or alternatively be coupled toportions of any of the leads of FIG. 1. The coil electrode 34, or othersimilar electrode types, can be electrically coupled to high voltagecircuitry for delivering high voltage cardioversion/defibrillation shockpulses. Electrodes shown in FIG. 1 can be disposed in a variety oflocations in, around, and on the heart.

The leads and electrodes described above can be employed to recordcardiac signals in the atria and the ventricles. The recorded data canbe periodically transmitted to a programmer or other external deviceenabled for telemetric communication with the medical device 10.

FIG. 2 is a functional block diagram of the implantable cardiac medicaldevice 10 shown in FIG. 1 according to one embodiment. Cardiac medicaldevice 10, referred to hereafter as ICD 10, generally includes timingand control circuitry 152 and a controller that may be embodied as amicroprocessor 154 or a digital state machine for timing sensing andtherapy delivery functions in accordance with a programmed operatingmode. Microprocessor 154 and associated memory 156 are coupled to thevarious components of ICD 10 via a data/address bus 155. ICD 10 includestherapy delivery module 150 for delivering electrical stimulation pulsesto a patient's heart including cardiac pacing pulses, arrhythmia pacingtherapies such as anti-tachycardia pacing (ATP) andcardioversion/defibrillation shocks, under the control of timing andcontrol 152 and microprocessor 154. Therapy delivery module 150 istypically coupled to two or more electrodes 168 via an optional switchmatrix 158. Electrodes 168 may include the electrodes shown in FIG. 1.Switch matrix 158 is used for selecting which electrodes andcorresponding polarities are used for delivering electrical stimulationpulses.

Cardiac electrical signals are sensed for determining when an electricalstimulation therapy is needed and in controlling a stimulation mode andthe timing of stimulation pulses. Electrodes used for sensing andelectrodes used for stimulation may be selected via switch matrix 158.When used for sensing, cardiac signals received by electrodes 168 arecoupled to signal processing circuitry 160 via switch matrix 158. Signalprocessor 160 includes sense amplifiers and may include other signalconditioning circuitry such as filters and an analog-to-digitalconverter. Cardiac electrical signals may then be used by microprocessor154 for detecting physiological events, such as detecting anddiscriminating cardiac arrhythmias. Signal processing circuitry 160 mayinclude event detection circuitry generally corresponding to P-wave orR-wave detection circuitry as disclosed in U.S. Pat. No. 5,117,824(Keimel, et al.), hereby incorporated herein by reference in itsentirety.

Arrhythmia detection algorithms may be implemented for detectingventricular tachycardia (VT), ventricular fibrillation (VF) as well asatrial arrhythmias such as atrial fibrillation (A FIB). Sensedventricular event intervals (R-R intervals) and sensed atrial eventintervals (P-P intervals) measured from the sensed cardiac signals arecommonly used for detecting and discriminating atrial and ventriculararrhythmias. Additional information obtained such as R-wave morphology,slew rate, other event intervals (e.g., P-R intervals) or other sensorsignal information may be used in detecting, confirming ordiscriminating an arrhythmia. Reference is made, for example, to U.S.Pat. No. 5,354,316 (Keimel), U.S. Pat. No. 5,545,186 (Olson et al.) andU.S. Pat. No. 6,393,316 (Gillberg et al.) for examples of arrhythmiadetection and discrimination using EGM signals and the provision ofarrhythmia therapies in response to arrhythmia detection anddiscrimination, all of which patents are incorporated herein byreference in their entirety.

In one detection scheme, programmable detection interval rangesdesignate the range of sensed event intervals indicative of atachycardia and may be defined separately for detecting slowtachycardia, fast tachycardia and fibrillation. In addition to eventinterval information, the morphology of the EGM signal may be used indiscriminating heart rhythms. As will be described herein, the cardiacsignal response to dual chamber burst pacing delivered in response todetecting both atrial and ventricular tachycardias can be used todiscriminate between SVT and VT. Microprocessor 154 may initiateatrial-ventricular pacing, referred to herein as dual chamber pacing,for use in tachycardia discrimination particularly when the sensedatrial and ventricular rates are so similar that other tachycardiadetection methods are not sensitive enough to discriminate between VTand SVT.

In response to arrhythmia detection, a programmed arrhythmia therapy isdelivered by therapy delivery module 150 under the control of timing andcontrol 152. Arrhythmia therapies may include tiered therapies in whichless aggressive ATP regimens are delivered first and, when notsuccessful, a high voltage shock therapy is delivered. A description ofhigh-voltage output circuitry and control of high-voltage shock pulsedelivery is provided in the above-incorporated '186 Olson patent.

ICD 10 may additionally be coupled to one or more physiological sensors170 carried by leads extending from ICD 10 or incorporated in or on theICD housing. Signals from sensors 170 are received by a sensor interface162 which provides sensor signals to signal processing circuitry 160.Sensor signals may be used by microprocessor 154 for detectingphysiological events or conditions.

The operating system includes associated memory 156 for storing avariety of programmed parameter values that are used by microprocessor154. The memory 156 may also be used for storing data compiled fromsensed physiological signals and/or relating to device operating historyfor telemetry out on receipt of a retrieval or interrogationinstruction. Parameters and tachycardia discrimination rules andalgorithms may be stored in memory 156 and utilized by microprocessor154 for controlling the delivery of dual chamber pacing anddiscrimination of detected tachycardia episodes based on the cardiacresponse to the dual chamber pacing. In one embodiment, memory 156stores a set of tachycardia discrimination rules relating to a heart'sresponse to dual chamber burst pacing.

A burst of pacing pulses, i.e. a programmed number of pacing pulsesdelivered at a rate faster than a detected tachycardia in both the atriaand the ventricles, is used for tachycardia discrimination. The pacingburst is not necessarily intended to terminate the tachycardia, unlikeprogrammed ATP regimes. In some cases, however, it is possible that theburst of pacing pulses does terminate the tachycardia. The burst ofpacing pulses are delivered at a fixed pacing pulse interval in theillustrative embodiments described herein, however it is conceivablethat dual chamber pacing pulses delivered for tachycardia discriminationmay be delivered at a progressively shortened interval, commonlyreferred to as “ramp” pacing, or any combination of fixed rate burstpacing pulses and ramp pacing pulses.

As will be described in greater detail herein, in response to detectinga tachycardia, microprocessor 154 will cause timing and control 152 toenable therapy delivery module 150 to deliver a burst of pacing pulsesin both the atria and the ventricles. Tachycardia discrimination rulesrelating to the heart's response to the dual chamber burst pacing willbe selected and applied by microprocessor 154 for discriminating thedetected tachycardia as SVT or VT.

ICD 10 further includes telemetry circuitry 164 and antenna 165.Programming commands or data are transmitted during uplink or downlinktelemetry between ICD telemetry circuitry 164 and external telemetrycircuitry included in a programmer or monitoring unit.

A recent technique to distinguish VT from SVT using dual chamber pacingdelivery is generally described in U.S. Pat. No. 7,206,633 (Saba).Briefly, simultaneous pacing is delivered in the atria and in theventricle (dual chamber pacing) after detecting tachycardia. Theearliest arriving electrical signal sensed following the dual chamberpacing is used to diagnose the detected tachycardia as SVT or VT. It isassumed that the chamber in which the fast rhythm is originating will beidentifiable by the earliest occurring intrinsic event subsequent todual chamber pacing. If the earliest intrinsic signal is sensed in theatrium after stopping the simultaneous dual chamber pacing, the detectedtachycardia is classified as SVT. If the earliest intrinsic signal issensed in the ventricle, the detected tachycardia is classified as VT.

While this particular simultaneous pacing technique can be used toeffectively differentiate between arrhythmias originating in theventricular and supraventricular regions of the heart in many cases,this technique has been found by the inventors to, at times, lead tofalse detections.

FIG. 3 is a timeline 100 of a VT episode falsely detected as SVT. Atachycardia episode 102 is detected having 1:1 correspondence betweenthe atrial sense events 106 (depicted as vertical lines along the upperportion of timeline 100) and the ventricular sense events 104 (depictedas vertical lines along the lower portion of timeline 100). In thiscase, the 1:1 tachycardia episode 102 is a ventricular tachycardia withretrograde conduction to the atrium represented by the upward diagonalarrows extending from ventricular sense events 104 to the resultingatrial sense events 106. An atrial refractory period 108 follows eachatrial sense event 106.

Simultaneous dual chamber pacing 110 is initiated in an attempt todiscriminate the 1:1 tachycardia 102 as ventricular or supraventricularin origin. A series of simultaneous atrial pacing pulses 112 andventricular pacing pulses 114 are delivered at a rate faster than thedetected tachycardia 102. The first atrial pacing pulse 112, however,occurs during the atrial refractory period 108′ following the lastatrial sensed event 106′. The total duration of the retrograde VAconduction time (not explicitly shown but generally indicated by theupward diagonal arrows) plus the atrial refractory period 108′ extendsafter delivery of the first atrial pacing pulse 112 such that the pacingpulse 112 is unable to capture atrial tissue.

As a result, for each ventricular pacing pulse 114, retrogradeconduction continues to cause atrial depolarizations represented by thedashed vertical lines during the dual chamber burst pacing. Each ofthese atrial depolarizations is followed by an atrial refractory period,which prevents atrial capture by the atrial pacing pulses 112.

After delivering the last ventricular pacing pulse 118, a retrogradeconducted atrial depolarization is sensed as atrial sense event 120.Thus, the earliest occurring sensed event 120 following dual chamberpacing 110 is an atrial event, signifying, incorrectly, that thetachycardia is originating in the atrium. The atrial sense event 120appears as an intrinsic event unassociated with a pacing pulse since itoccurs after the dual chamber pacing 110 has stopped. The origin of thefast rhythm is thus falsely detected as supraventricular.

The situation shown in FIG. 3 generally arises when a patient has arelatively long atrial refractory period and/or a relatively longretrograde conduction time. When the patient has a relatively shortatrial refractory period, the refractory period 108′ may be over beforethe dual chamber burst pacing 110 is initiated, resulting in propercapture of the atrium during dual chamber pacing 110. However, since theatrial refractory period of the patient may be unknown, it is desirableto provide a method for delivering dual chamber pacing that results inmore reliable discrimination of SVT and VT in all patients.

FIG. 4A is a timing diagram 150 illustrating one method for deliveringdual chamber burst pacing in the atrial and ventricular regions of apatient's heart for discriminating SVT and VT. As used herein, “dualchamber” pacing refers to pacing delivered in both atrial andventricular regions. Dual chamber burst pacing is delivered in one orboth atria and in one or both ventricles. The interval between an atrialpacing pulse and a ventricular pacing pulse during the dual chamberburst pacing is referred to as the atrial-ventricular pacing interval(AVI).

Pacing techniques described herein are provided via a controller withinthe IMD 10, such as the microprocessor 154 operating in conjunction withmemory 156 and timing and control module 152 in FIG. 2. A controller mayalternatively be implemented using one or more digital state machines. Acontroller providing the functionality described herein may includefunctionality distributed across more than one cardiac medical devicecomponent.

The timing diagram 150 of FIG. 4A includes atrial paced and sensedevents shown along the top line (A) and ventricular paced and sensedevents along the bottom line (V). The sensed and paced atrium caninvolve either or both of the patient's atria and likewise, the sensedand paced ventricle can involve either or both of the patient'sventricles.

Similar to FIG. 3, a tachycardia episode 152 is detected having 1:1correspondence between atrial and ventricular sensed events 156 and 154respectively. The tachycardia episode 152 is a ventricular tachycardiawith retrograde conduction to the atrium (as indicated by the upwarddiagonal arrows) causing an atrial sense event 156 for each ventricularsense event 154. However, because the atrial event intervals and theventricular event intervals are very similar and have 1:1correspondence, the tachycardia episode 152 is difficult to discriminateusing interval-based criteria.

Each atrial event 156 occurs after each ventricular event 154 followinga retrograde, ventricular-atrial (VA) conduction time 158. Theretrograde conduction time 158 may vary from patient to patient and evenwithin a given patient when different tachycardia episodes havedifferent ventricular sites of origin.

Dual chamber burst pacing methods described in conjunction with FIG. 4Aaddress the situation described above in conjunction with FIG. 3 bydelivering a single chamber pacing pulse 170 preceding the dual chamberpacing 160. The single chamber pacing pulse 170 is delivered in theatrium prior to the dual chamber atrial and ventricular pacing pulses162 and 164, respectively, by a predetermined interval, also referred toherein as the “prematurity” of the single chamber pacing pulse.

FIG. 4B is a timing diagram showing a portion of the timeline of FIG. 4Ain greater detail. The single chamber pacing pulse prematurity 190, alsoreferred to as the premature paced AA (PAA) interval, may be set to adefault value, for example approximately 150 ms, earlier than ascheduled dual chamber atrial pulse 164. Alternatively, a desired PAA190 may be computed, as will be further described below in conjunctionwith FIG. 7.

Several requirements may be imposed on the timing of premature pacingpulse 170 to promote delivery of the premature pulse 170 at a time thatwill likely capture the atria and promote capture of the atria by thefirst dual chamber pulse 164. One requirement is that the prematurepulse 170 is scheduled outside of the atrial refractory period 157following the last atrial sense event 156′. Another requirement is thatPAA 190 is longer than the atrial refractory period 157′ that followspremature pulse 170 so that the first dual chamber pulse 164 fallsoutside refractory period 157′. The timing of the premature pulse 170should meet these requirements while still occurring earlier than a nextexpected atrial sense event 182 which is intended to be blocked bypremature pulse 170.

For example, as shown in FIG. 4B, the premature pulse 170 may bedelivered a specified time interval 186 earlier than an expected atrialsense event 182. The expected atrial sense event 182 is the nextintrinsic atrial event that would normally occur after sensed event 156′and prior to the first dual chamber atrial pacing pulse 164 if thesingle chamber pulse 170 were not delivered. The expected atrial senseevent 182 is expected to occur at approximately the sensed atrial cyclelength 188. In one embodiment, the premature atrial pacing pulse 170 isdelivered at approximately 50 ms earlier than the expected atrial senseevent 182. The timing of the premature pulse 170 may be based on settingan escape interval timer starting at the last atrial sensed event 156′at an interval that is equal to the sensed atrial event interval 188less the fixed interval 186. In this way, the premature pacing pulse 170captures the atrium during the retrograde conduction time following thelast ventricular sense event 155 and thereby prevents the expectedatrial sense event 182 from occurring.

In practice, the timing of the dual chamber pacing pulses 162 and 164 isgenerally based on the last ventricular sensed event 155 to avoidinducing ventricular tachycardia. The first ventricular pacing pulse isscheduled at a selected pacing cycle length (PCL) 180 following the lastventricular sense event 155. The first dual chamber atrial pacing pulse164 is scheduled at an AV interval 192 relative to the first ventricularpacing pulse 162 (PCL 180 minus AV interval 192). Knowing the timing ofthe first dual chamber pacing pulses 162 and 164 relative to the lastventricular sense event 155, a paced ventricular atrial (PVA) interval196 may be computed for controlling delivery of the atrial prematurepulse 170 using a pacing time interval set upon the last ventricularsense event 155 and meeting the required bounds for promoting capture ofboth the premature pulse 170 and the first dual chamber pulse 164.Additional details regarding decision steps and computations made forcontrolling the delivery of premature pulse 170 will be discussed belowin conjunction with FIGS. 6 and 7.

The pacing cycle length 180 is shown relative to the last ventricularsensed event 155 and the first ventricular pacing pulse 162. As such,the timing of the first atrial and ventricular pulses 164 and 162 of thedual chamber burst pacing is based on the occurrence of the lastventricular sensed event 155. This ventricular-based timing of the dualchamber burst of pacing pulses is provided to avoid inducing VT. Whiledual chamber pacing could conceivably be timed relative to a last atrialsensed event, the timing of the dual chamber pacing should be set toavoid inducing ventricular arrhythmias by controlling the ventricularcoupling interval between the last ventricular sensed event 155 and thefirst ventricular pacing pulse 162.

Referring again to FIG. 4A, the premature atrial pacing pulse 170 isintended to capture (pre-excite) the atrium prior to the end of theretrograde conduction time 158′ following a last ventricular sense event155. The single-chamber atrial pacing pulse 170 will depolarize theatrium to cause an atrial refractory period 165. In this way, thesingle-chamber pacing pulse 170 blocks depolarization of the atrium dueto the retrograde conduction of the last ventricular sense event 155.The downward diagonal arrow extending from premature pacing pulse 170suggests the normal antegrade conduction of the atrial depolarizationtoward the ventricle. The upward diagonal arrow extending from the lastventricular sense event 155 indicates the retrograde conduction of theventricular depolarization. The meeting of the two arrows schematicallyillustrates that the propagation of the atrial depolarization (antegradeconduction) is blocked by the ventricular refractory period as theventricular depolarization propagates retrograde. Likewise, thepropagation of the ventricular depolarization (retrograde conduction) isblocked by the atrial refractory period as the atrial depolarizationpropagates antegrade.

The single chamber atrial pacing pulse 170 can be thought of as apremature atrial pacing pulse as it is delivered a short interval priorto the first dual chamber pulses 162 and 164, at a selected prematurity,thus resulting in two consecutive atrial pacing pulses without anintervening ventricular pace or sense. The burst pacing interval 180 isthe interval between the last ventricular sensed event 155 and the firstdual chamber ventricular pulse 162 and between each of the subsequentdual chamber pacing pulses. The AVI during dual chamber pacing may beset to 0 ms for simultaneous pace pulses delivered in the atria andventricles or to some minimal, non-zero interval (typically less thanapproximately 60 ms) to provide dual chamber burst pacing in the atriaand ventricles.

The initial premature atrial pace 170 blocks the retrograde conductionof the final intrinsic ventricular event 155 such that subsequent atrialpacing pulses 164 are delivered outside of the atrial refractory period165 with sufficient pulse energy to capture the atrium. These atrialpacing pulses 164 block retrograde conduction of the simultaneousventricular pacing pulses 162. Following the final dual chamber pacingpulse 172, the first intrinsic event 174 that is sensed after dualchamber pacing 160 is a ventricular event. The fast ventricular ratereappears first after termination of the dual chamber pacing 160signifying that the tachycardia is originating in the ventricle. Thefast ventricular rate is again conducted retrograde to the atrium tocause atrial events 176 at a 1:1 correspondence with the intrinsicventricular events 174, however, the conducted atrial events occursafter the earliest occurring sense event in the ventricle. Thus byproviding a premature atrial pacing pulse prior to the dual chamberpacing burst, the earliest occurring intrinsic sensed event 174correctly signifies the chamber of tachycardia origin.

Illustrative embodiments presented herein describe an atrial prematurepacing pulse preceding a series of dual chamber pacing pulses. It iscontemplated that in alternative embodiments, a premature ventricularpacing pulse may be delivered additionally or alternatively to thepremature atrial pacing pulse.

FIG. 5 is a flow chart of a method for delivering dual chamber pacingfor use in discriminating SVT and VT. Flow chart 200 is intended toillustrate the functional operation of the device, and should not beconstrued as reflective of a specific form of software or hardwarenecessary to practice the methods described. It is believed that theparticular form of software will be determined primarily by theparticular system architecture employed in the device and by theparticular detection and therapy delivery methodologies employed by thedevice. Providing software to accomplish the described functionality inthe context of any modern cardiac medical device, given the disclosureherein, is within the abilities of one of skill in the art.

Methods described in conjunction with flow charts presented herein maybe implemented in a computer-readable medium that includes instructionsfor causing a programmable processor to carry out the methods described.A “computer-readable medium” includes but is not limited to any volatileor non-volatile media, such as a RAM, ROM, CD-ROM, NVRAM, EEPROM, flashmemory, and the like. The instructions may be implemented as one or moresoftware modules, which may be executed by themselves or in combinationwith other software.

Method 200 is initiated upon detecting tachycardia at block 202. Initialtachycardia detection may be made utilizing event interval, morphologyor pattern based criteria or any combination thereof. At initiatingblock 202, detection of 1:1 correspondence in atrial and ventriculartachycardia events initiates the use of dual chamber pace discriminationmethods described herein. For example, a 1:1 tachycardia may be detectedwhen the atrial cycle lengths (P-P intervals) and ventricular cyclelengths (R-R intervals) both meet tachycardia detection criteria and areapproximately equal, e.g., within approximately 30 ms of each other withone atrial event and one ventricular event occurring during each cycle.If a detected tachycardia does not have a 1:1 correspondence betweenatrial and ventricular sensed events, interval, morphology, and/orpattern based tachycardia discrimination methods may be used to classifythe tachycardia, e.g. as SVT, VT, atrial fibrillation, dual tachycardiaor sinus tachycardia. Examples of pattern-based methods for tachycardiadiscrimination are generally disclosed in the above-referenced '186Olson patent.

When the atrial and ventricular cycle lengths are substantially equal,interval or pattern based rhythm classification methods may not be ableto reliably discriminate VT from SVT. Dual chamber pacing is initiatedto allow discrimination methods to be performed based on the heart'sresponse to pacing. The dual chamber pacing may additionally serve toterminate the tachycardia.

As described above, in some embodiments burst pacing is initiated with asingle, premature atrial pacing pulse at block 204. The single atrialpacing pulse is delivered at a predetermined interval before the firstatrial pacing pulse of the dual chamber pulses. The single atrial pacingpulse is delivered at an interval defined relative to the first dualchamber atrial pacing pulse or relative to the last intrinsic, sensedventricular event before starting the dual chamber pacing. In eithercase, the single atrial pacing pulse is delivered at a time intervalthat blocks retrograde conduction of the last sensed ventricular event.

The premature pacing pulse is delivered at a prematurity interval thatis at least greater than the atrial refractory period such that thefirst atrial pulse of the subsequent dual chamber pulses is able tocapture the atrium (falls outside the atrial refractory periodassociated with the premature pulse). The premature atrial pacing pulseis delivered after the last ventricular sensed event at an interval thatis less than the retrograde conduction time in order to capture theatrium and thereby block retrograde conduction of the ventriculardepolarization to the atrium.

At block 206, dual chamber pacing is delivered at a pacing cycle lengthselected to be less than the detected tachycardia cycle length. Dualchamber pacing directly follows the single chamber pacing pulse. Thefirst dual chamber pacing pulses follow the single chamber pacing pulseby a time interval that exceeds the atrial refractory period such thatthe atrial pacing pulses capture the atrium. The dual chamber pacingcycle length may be set as a percentage of the tachycardia cycle length.The atrial and ventricular pacing pulses may be delivered substantiallysimultaneously, e.g. with a 0 ms AV interval though non-zero intervalsmay be used such as intervals less than approximately 60 ms. Apredetermined number of dual chamber pulses may be delivered. Typicallyfive to eight pacing cycles are delivered but methods described hereinare not limited to a specific number of dual chamber pulses.

After delivering the dual chamber pacing pulses, the chamber in whichthe earliest occurring intrinsic event is sensed after the last pacingpulse is identified at block 208. The sensed event is an intrinsicdepolarization, not an evoked response to the pacing pulses. The firstchamber eliciting an intrinsic depolarization corresponds to the chamberin which the fast rate is originating. If the first chamber is an atrialchamber, i.e. the first sensed event is a P-wave as determined at block210, the tachycardia episode is classified as SVT at block 212. If thefirst event is a ventricular event, i.e. an R-wave, the tachycardiaepisode is classified as VT at block 214.

FIG. 6 is a flow chart of a method 300 for discriminating VT and SVTaccording to an alternative embodiment. In the following description ofFIG. 6, reference is also made to the timing diagram shown in FIG. 7 tomore clearly illustrate computations and decisions being made in method300. FIG. 7 corresponds to the detailed timing diagram of FIG. 4 withadditional intervals indicated. Briefly, a last atrial sense event 156′and last ventricular sense event 155 are shown followed by the atrialpremature pulse 170, and the first dual chamber pulses 164 (atrial) and162 (ventricular) separated by a paced AVI 192. The premature atrialpacing pulse 170 is delivered at a PVA interval 196 following the lastventricular sense event 196.

Referring to FIG. 6, at block 302 tachycardia is detected having 1:1correspondence in the atrial and ventricular chambers. Blocks 304 and306 are then performed to determine whether to deliver a prematureatrial pacing pulse to pre-excite the atrial chambers before initiatingdual chamber burst pacing.

At block 304, a time interval between a last expected atrial sense event(if no premature pulse is delivered) and the first dual chamber atrialpacing pulse is computed. This time interval, referred to as the atrialsense-atrial pace (ASAP) interval is shown in the timing diagram of FIG.7. The ASAP interval 184 between an expected atrial sense event 182 andthe first atrial pacing pulse 164 of the dual chamber pacing is computedat block 304 as the pacing cycle length (PCL) 180 minus the VA interval158′ minus the paced AVI 192.

If the ASAP interval is greater than a predetermined limit, for examplegreater than 150 ms, then an atrial pacing pulse 164 following theexpected atrial sense event 182 should be late enough after the expectedatrial sense event 182 to occur outside atrial refractory. In this case,the first dual chamber pulse 164 is expected to be able to capture theatria. The premature atrial pacing pulse is not needed. In FIG. 6,method 300 advances directly to block 312 to deliver the dual chamberburst pacing for a desired number of pacing cycles without delivering apremature atrial pacing pulse.

If the ASAP interval 184 is less than 150 ms, there is a chance that thefirst atrial pacing pulse 164 of the dual chamber burst pacing will notcapture the atria. A premature, single atrial pacing pulse 170 will bedelivered to block the expected atrial sense event 182 that wouldotherwise precede the first atrial pacing pulse 164 by less thanapproximately 150 ms, or another predetermined limit corresponding to anestimated atrial refractory period 157′.

At block 308, computations are made to determine PVA interval 196 usedto control the delivery of premature pulse 170 relative to the lastventricular sensed event 155. First a desired prematurity, i.e. PAAinterval 190, between the premature atrial pacing pulse 170 and thefirst dual chamber atrial pacing pulse 164 is determined using the firstequation in block 308. The desired PAA 190 may be computed as the ASAPinterval 184 plus a predetermined fixed interval (e.g. approximately 50ms) so that the premature atrial pulse 170 is delivered before orearlier than the expected atrial sense event 182. PAA 190 may beselected as the maximum of a selected lower limit, for example 150 ms,or the ASAP interval 184 plus a fixed interval (e.g. 50 ms) whichever isgreater, as shown by the first equation in block 308.

A selected lower limit for PAA interval 190 prevents the prematureatrial pacing pulse 170 from being delivered at a PAA interval 190 thatis less than an expected atrial refractory period 157′. Otherwise, thefirst atrial pacing pulse 164 of the dual chamber burst pacing could bedelivered during the physiological refractory period 157′ following thepremature pulse 170 and fail to capture the atria. As such, in oneembodiment, the desired PAA interval 190 is 150 ms or the computed ASAPplus 50 ms, whichever is greater. PVA interval 196 can then computed asthe selected pacing cycle length 180 less the programmed AVI 192 lessthe desired PAA 190.

Alternatively, the PVA interval 196 is computed as the differencebetween the sensed VA interval 158′ and an interval 186 at which theatrial pacing pulse 170 will precede an expected atrial sense event 182,referred to as the atrial pace-P interval or “APP” interval 186. APP 186is computed at block 308 according to the second equation as thedifference between PAA 190 and the computed ASAP interval 184. In orderto block a retrograde atrial sensed event, the premature atrial pacingpulse should be delivered earlier than the expected atrial sense event182 but within an atrial refractory period 158′ of the expected atrialsense event 182. For example, APP 186 may be set to a fixed interval ofapproximately 50 ms, as described in conjunction with FIG. 4B. In otherembodiments, APP 186 may be variable, e.g. if the PAA interval is set toa fixed interval, e.g. 150 ms. In this case, the APP 186 is computed asthe PAA interval 190 less the ASAP interval 184 as shown by the secondequation in block 308. The PVA interval 196 used to control the deliveryof the single atrial premature pulse 170 using an escape intervalstarted upon the last ventricular sense event 155 is computed as thedifference between the measured VA interval 158′ and the APP interval186, as shown by the third equation in block 308.

It is recognized that numerous variations in timing schemes may beconceived for controlling delivery of the atrial premature pulse. Ingeneral, the atrial premature pulse 170 should be more than one atrialrefractory period 157′ earlier than the first dual chamber pulse 164 (toensure capture by the dual chamber pulse) and more than one atrialrefractory period 157 later than the last atrial sense event 156′ (topromote capture by the premature pulse 170), and within the VA interval158′ (to prevent the atrial sense event 182 from occurring).

At block 309, and with continued reference to FIG. 7, the timing of thepremature atrial pacing pulse 170 relative to the last atrial senseevent 156′ is checked to ensure the premature pulse 170 is highlyunlikely to fall within the physiologic refractory period 157 of thelast atrial sense event 156′. The sum of the sensed AV interval 197 andthe computed PVA interval 196 equals interval 198 between the lastatrial sensed event 156′ and the scheduled premature atrial pacing pulse170. This interval 198 should be greater than the atrial refractoryperiod 157 to ensure that premature pulse 170 does not fall within theatrial refractory 157 and fail to capture the atria.

As such, at block 309 of method 300, a comparison between the sum of thesensed AV interval 197 and the computed PVA interval 196 is compared toa selected interval that is equal to or greater than an estimated atrialrefractory period, e.g. 150 ms. In other embodiments, the atrialrefractory period may be estimated to be approximately 100 ms oranywhere between approximately 100 ms and 150 ms.

If the premature atrial pacing pulse will occur too early following thelast atrial sensed event based on the determination at block 309, method300 proceeds to block 312 and delivers dual chamber pacing withoutdelivering a premature atrial pacing pulse. If the premature atrialpacing pulse 170 is likely to be outside of atrial refractory 157 usingthe computed PVA 196, according to the comparison at block 308, thepremature atrial pacing pulse 170 is delivered at block 310 using thecomputed PVA 196 and is followed by the dual chamber burst pacing atblock 312.

The dual chamber burst pacing is delivered for a desired number ofcardiac cycles at a pacing cycle length that is shorter than thedetected tachycardia cycle length, for example less than approximately90% of the tachycardia cycle length. In some embodiments, the pacingcycle length is approximately 80% of the tachycardia cycle length. Aselected AVI is applied during the dual chamber burst pacing.

At block 314, a tachycardia discrimination blanking period may beapplied following the last dual chamber pacing pulses. In some cases, apremature contraction may be the earliest occurring intrinsic sensedevent following the dual chamber pacing. As such, a blanking period maybe applied to one or both of the atrial and ventricular sensed events bythe ICD controller immediately following the respective atrial andventricular pacing pulses to prevent identification of a prematurecontraction as the earliest occurring event signifying the origin of thetachycardia.

This tachycardia discrimination blanking period may be set toapproximately equal the detected tachycardia cycle length. As such, thisblanking period is generally longer than a blanking period commonlyapplied to sense circuitry to prevent improper sensing of pacing pulseartifact or other noise during atrial or ventricular refractory periodsto avoid sensing noise or artifact as cardiac intrinsic events. Thetachycardia discrimination blanking period set to an intervalapproximately equal to the detected tachycardia cycle length (i.e. thesensed PP or sensed RR intervals during the detected tachycardia episodeprior to initiating dual chamber pacing), or to an interval slightlyless than the tachycardia cycle length, will be relatively longer thanthe absolute blanking periods typically applied to sense circuitry toavoid sensing artifact or noise following a pacing pulse or duringphysiological refractory period.

The tachycardia discrimination blanking period is used to identifyevents that occur at a cycle length that is shorter than theoriginally-detected tachycardia cycle length. A sensed event occurringwithin the tachycardia discrimination blanking period is not relied uponalone for signifying the cardiac chamber of origin of theoriginally-detected tachycardia episode. The use of a tachycardiadiscrimination blanking interval is illustrated in FIG. 8.

FIG. 8 is a timeline 350 of a detected tachycardia 352 having 1:1correspondence between ventricular sensed events 354 and atrial sensedevents 356. Dual chamber burst pacing 360 is delivered at a rate fasterthan the detected tachycardia rate.

Upon terminating the dual chamber burst pacing 360, the earliestoccurring intrinsic sensed event 370 occurs in the ventricle. In thisexample, the earliest occurring event 370 is a premature ventricularcontraction (PVC). The PVC 370 occurs at an interval 372 after the lastventricular pacing pulse 368 that is shorter than the detectedtachycardia cycle length 358. Atrial sensed events occurring after dualchamber burst pacing 360 return at the tachycardia cycle length 362. Inthis example, the detected tachycardia 352 originates in the atrialchambers but may be falsely detected as VT due to the PVC 370 sensed asthe earliest-occurring intrinsic event after dual chamber burst pacing360.

To avoid this false detection of VT, a blanking interval 380 is applieddirectly following the dual chamber burst pacing 360. The blankinginterval 380 may be set approximately equal to the detected tachycardiacycle length 362. Blanking interval 380 may be set slightly less thancycle length 362 to account for some variation in cycle length. Byblanking or ignoring any events sensed at an interval shorter than thedetected tachycardia cycle length, events such as premature contractionsthat are not associated with the originally-detected tachycardia can beignored.

In the illustrative example of FIG. 8, a PVC 370 is shown as theearliest occurring sensed event. However, a premature atrial contraction(PAC) may similarly cause a false detection of SVT when VT is actuallypresent and a PAC occurs as the earliest sensed event following dualchamber burst pacing 360. In various embodiments, a tachycardiadiscrimination blanking interval 380 may be applied to atrial sensedevents, to ventricular sensed events or both atrial and ventricularevents to prevent premature contractions, retrograde conducteddepolarizations, or other events occurring earlier than the detectedtachycardia cycle length from causing false classification of thetachycardia.

When a PVC 370 occurs following dual chamber burst pacing 360, thesubsequently sensed events commonly exhibit a pattern of VAAV 364. Inother words, a sensed event 370 in the ventricles is followed by twoatrial sensed events and then another ventricular sensed event. Thispost-pace VAAV pattern 364 may also be used to discriminate between SVTand VT. When a VAAV pattern 364 occurs following the dual chamberpacing, the first V event is likely a PVC followed by the atrial eventscorresponding to an atrial tachycardia, particularly when the earliestoccurring sense event occurs during a tachycardia discriminationblanking interval. This VAAV pattern and other pattern analysis usefulin paced tachycardia discrimination methods will be further discussedbelow.

Returning to FIG. 6, the chamber in which the earliest intrinsic, sensedevent occurs is identified at block 318. When the blanking interval isapplied at block 314, this earliest occurring event will be outside thetachycardia discrimination blanking interval, i.e. a non-blanked event.In some embodiments, any earlier events, falling within the tachycardiadiscrimination blanking period, are ignored for the purposes ofdiscriminating the tachycardia.

Alternatively, as will be described further below, blanking intervalevents are used in discrimination algorithms to evaluate post-pacesensed event patterns. In either case, however, the blanking period isused to distinguish between intrinsic, sensed events that correspond tothe rate and chamber of origin of the detected tachycardia and sensedevents that do not correspond to either the rate or the chamber oforigin of the detected tachycardia. In other words, an earliestoccurring event within the tachycardia discrimination blanking intervalcannot be used alone in discriminating the tachycardia. At least oneadditional event outside the blanking interval is required for reliablydiscriminating the tachycardia.

If the earliest occurring non-blanked event is in the atrial chamber, asdetermined at block 320, the tachycardia is detected as SVT at block322. The ICD may deliver an atrial ATP therapy according to a menu oftherapies. SVT detection will prevent ventricular ATP therapies frombeing delivered unnecessarily.

If the earliest occurring non-blanked event is in the ventricularchamber (a negative result at block 320), the timing of the earliestventricular sense event relative to the next atrial sense event may beevaluated at block 324 before detecting the tachycardia as VT at block326. For example, if the ventricular sense event is at least somepredetermined interval earlier than the first atrial sense event(non-blanked), e.g. approximately 50 ms earlier, the tachycardia isdetected as VT (block 326). An ATP therapy may be delivered to treat theVT according to programmed therapies.

If the earliest occurring ventricular sense event is less than 50 msearlier than the earliest occurring atrial sense event, the tachycardiais likely to be SVT, but the two events may be too close together toreliably discriminate between VT and SVT. As such, SVT may bepreliminarily detected at block 328, preventing or delaying anyscheduled VT therapy, and the dual chamber pace discrimination methodmay be repeated by returning to block 312 to confirm the SVT detection.

It is recognized that repeating the dual chamber pace discriminationmethod by returning to block 312 to confirm a preliminary detection mayinclude delivering a premature atrial pacing pulse if a PVA interval canbe utilized that meets the timing requirements of the premature pacingpulse relative to the last atrial sensed event and the first dualchamber atrial pacing pulse as described previously. In someembodiments, if a premature single chamber pacing pulse was included inthe initial attempt at dual chamber pace discrimination, the prematuresignal chamber pacing pulse may be excluded on a repeated attempt toallow comparisons of the post-pacing response with and without thepremature pulse. Likewise, if the premature pulse was not includedduring the initial burst pacing, a premature pacing pulse may be addedin repeated attempts as long as a premature pacing interval does notresult in the pacing pulse being delivery during an expected refractoryperiod of the last atrial sense event.

If a premature single chamber pacing pulse is delivered during repeatedpace discrimination attempts, it may be delivered at a differentprematurity interval than an initial premature pulse interval. Differentprematurity intervals may be provided by varying the prematurity by afixed interval from a computed PVA, e.g. between approximately 10 to 50ms increments or decrements. Different prematurity intervals mayalternatively be provided by varying the fixed interval added to theASAP interval in the first equation of block 308 used for computing adesired prematurity. The PVA may be varied as long as the requirement atblock 309 continues to be met.

FIG. 9 is a flow chart 350 of an alternative method for pacedtachycardia discrimination. In FIG. 9, identically numbered blockscorrespond to those shown in FIG. 6. In the method shown in FIG. 9,after identifying the chamber in which the earliest non-blanked sensedevent occurs, a morphology analysis may be performed for discriminatingVT and SVT.

If the earliest occurring event is an atrial event (block 320)indicating SVT, the ventricular signal may be analyzed to support theSVT detection. If the morphology substantially matches a known normalmorphology corresponding to a ventricular depolarization conducted fromthe atrial chambers, as determined at block 352, SVT is detected atblock 354. A variety of morphology analysis methods may be used whichtypically include comparisons between the unknown ventricular signal anda known template. Reference is made, for example, to theabove-incorporated '316 Gillberg patent.

If the ventricular signal morphology does not substantially match aknown normal morphology corresponding to a conducted ventricular event,the dual chamber pacing discrimination method may be repeated byreturning to block 312 (which may include delivering a premature atrialpacing pulse as described above).

If the earliest occurring event is a ventricular event and is sensedless than 50 ms earlier than the earliest atrial event (block 356), therhythm is most likely SVT. Morphology analysis is performed at block 352to confirm a normal ventricular signal morphology before making the SVTdetection at block 354. If the earliest occurring event is a ventricularevent that is more than 50 ms earlier than the earliest atrial event(block 356), the tachycardia is most likely ventricular in origin.However, morphology analysis may be performed at block 358 to supportthis conclusion. If the ventricular signal morphology is determined tomatch a normally-conducted depolarization signal morphology, the resultsof the morphology analysis and the earliest occurring event beingventricular and greater than 50 ms earlier than the next atrial eventprovide inconsistent or conflicting results. The paced discriminationmethod may be repeated by returning to block 312.

A ventricular signal morphology that does not match a known morphologyfor a normally conducted ventricular depolarization corroborates theevidence for VT based on the earliest occurring event being aventricular event greater than 50 ms earlier than the next atrial event.VT is detected at block 360. Morphology analysis may thus beincorporated into the paced discrimination methods to confirm anevent-based discrimination or to cause pace discrimination methods to berepeated until non-conflicting results are obtained, the tachycardia isterminated or a maximum number of attempts or other time limit isreached. Some maximum limit may be applied to the number of dual chamberpacing episodes that are attempted to discriminate the tachycardiabefore a therapy delivery decision is made.

FIG. 10 is a flow chart 400 of a method for discriminating tachycardiausing additional analysis of the post-paced event pattern. At block 402,dual chamber burst pacing is delivered for a desired number of cardiaccycles (N). The delivery of dual chamber burst pacing may include apremature atrial pulse as described above. At block 404, a post-pace,tachycardia discrimination blanking period is applied to separate eventshighly likely to be indicative of the chamber of tachycardia origin andevents that are highly unlikely to be indicative of theoriginally-detected tachycardia. An event occurring within thetachycardia discrimination blanking period is occurring at a cyclelength that is shorter than the originally detected tachycardia cyclelength as described previously.

At block 406, a determination is made whether the earliest occurringintrinsic event after the dual chamber burst pacing is sensed during thetachycardia discrimination blanking interval. If the earliest event is anon-blanked ventricular sense event that is a predetermined minimuminterval earlier than the earliest atrial sense event (block 408), thetachycardia is detected as VT. If the earliest event is a non-blankedventricular sensed event less than the predetermined minimum interval(block 408), SVT is detected at block 410. As described above,additional analysis, such as morphology analysis or repeated dualchamber burst pacing may be delivered to allow confirmation of the SVTdetection at block 410.

When the earliest event occurs during the tachycardia discriminationblanking interval, as determined at block 406, additional analysis ofthe subsequent sensed event pattern is performed to discriminate thetachycardia. If the earliest occurring event is a blanked atrial eventas determined at block 414, an analysis of the next cardiac cycles isperformed at block 418.

If an AAV pattern is present within the next cycles, for example withinthe next five cardiac cycles, which may or may not include the cyclebeginning with the blanked atrial event, SVT is detected at block 420.This situation is illustrated in the inset timeline 432. A blankedatrial event is likely to be a premature atrial contraction which isconducted to the ventricle causing the earliest occurring event outsidethe blanking interval to be a ventricular event. A tachycardia detectionbased on the first non-blanked event alone, therefore, would potentiallyclassify the tachycardia as VT. However, the subsequent AAV pattern isevidence of SVT and the pattern analysis triggered by the blanked atrialevent allows a correct SVT detection to be made at block 420.

If no AAV pattern is present within the next N intervals (block 418),the earliest ventricular event outside the blanking interval correctlyindicates VT as detected at block 422. The absence of the AAV pattern asdepicted in the inset timeline 434 indicates that the fast ventricularrate appearing first after the dual chamber pacing and the tachycardiadiscrimination blanking interval is indeed associated with VT. Theblanked atrial event was likely to be a premature atrial contraction andnot indicative of the chamber of origin of the originally-detectedtachycardia.

When the blanked event is a ventricular event (negative result at block414), the next cycles may also be analyzed to detect the presence of theAAV pattern indicative of SVT. If present, SVT is detected at block 424.In this situation, illustrated by the inset timeline 436, the blankedventricular event is likely to be a premature ventricular contractionfollowed by the reappearance of a fast atrial rate.

If the AAV pattern is not present following a blanked ventricular event,morphology analysis is performed at block 426. This situation isdepicted in the inset timeline 438. In this case, the 1:1 correspondenceof atrial and ventricular events may correspond to an SVT with theblanked ventricular event being a premature ventricular contractionfollowed by a retrograde-conducted atrial event and this VAVA patterncontinues after the blanking interval. Alternatively, the blankedventricular event may be a premature ventricular contraction followed bythe reappearance of a fast atrial rate conducted 1:1 to the ventricles.As such, the event analysis alone is not sufficient to discriminate thetachycardia.

Morphology analysis of ventricular sense events is performed at block426 on one or more non-blanked ventricular sense events. If theventricular signal morphology substantially matches a known morphologycorresponding to a normally conducted ventricular depolarization, SVT isdetected at block 428. If the ventricular signal morphology isdetermined to not be associated with a normally conducted ventriculardepolarization, i.e., likely to originate in the ventricles, VT isdetected at block 430. Alternatively or additionally, pacediscrimination attempts may be repeated to confirm a result.

In summary, if an earliest occurring event following dual chamber pacingfalls within a tachycardia discrimination blanking interval, which isset based on the detected tachycardia cycle length, additional eventpattern analysis and/or morphology analysis may be invoked to evaluatesubsequent, non-blanked events, to classify the tachycardia.

FIG. 11 is a flow chart 500 of an alternative method for dual chamberpacing discrimination of tachycardia. In method 500, dual chamber burstpacing is delivered at block 502 for a desired number of cardiac cyclesas described previously without applying a tachycardia discriminationblanking interval. A premature atrial pacing pulse may or may not beapplied. At block 504, a desired number of post-pace ventricular cycles(N) are identified. The N ventricular cycles are identified byidentifying the earliest-occurring ventricular sense event after thelast dual chamber pacing pulse and the subsequent N+1 ventricular senseevents.

At block 505, the number of atrial sense events occurring during the Nventricular cycles is determined. The number of atrial sensed events isthen compared to the number of ventricular cycles N in subsequentdecision blocks 506, 510 and 514. When the number of sensed atrialevents is less than N−1, VT is detected at block 508. The greater numberof ventricular events is indicative of a fast rate originating in theventricular chambers. Only one ventricular cycle more than the number ofatrial events may be associated with a PVC, and is thus would not besufficient evidence to detect VT. Accordingly, the requirement to detectVT at block 508 is that there is less than N−1 atrial events during theN ventricular cycles as determined at block 506.

If there are more atrial events than ventricular cycles, i.e. more thanN atrial sense events as determined at block 510, SVT is detected atblock 512. The greater number of atrial events during N ventricularcycles indicates a fast atrial rate and is evidence of tachycardiaoriginating in the supraventricular region of the heart.

If the number of atrial sense events is exactly equal to N−1, (anegative result at each of decision blocks 506, 508 and 514), the singleventricular event not associated with an atrial sense event may be apremature ventricular contraction. On the other hand, one moreventricular event than atrial events may caused by a VT with 1:1retrograde conduction, i.e. every atrial sensed event is conductedretrograde from the ventricles. Additional analysis is need todiscriminate between these two conditions.

In one embodiment, the VA interval range is determined at block 530 andcompared to a threshold. The VA interval range can be determined bycomputing the VA interval for each of the N cycles. The maximum VAinterval and the minimum VA interval for the N cycles are thenidentified. The VA interval range is the difference between the maximumand minimum VA intervals. It is recognized that other computations maybe made to determine a metric of the range or variability of VAintervals occurring during the N ventricular cycles. For example, insome embodiments, only a maximum VA interval may be evaluated.

If the VA interval range is relatively small, for example less thanapproximately 30 ms, VT is detected at block 532. If the VA intervalrange is large, e.g., exceeds a threshold of 30 ms or another selectedthreshold value, SVT is detected at block 534. When the VA intervalrange is relatively small, VT is indicated because atrial eventsoccurring at a regular interval following ventricular events likelyrepresents retrograde conduction of the fast ventricular rhythm to theatria.

A relatively large VA interval range is not typical during VT andindicates the atrial and ventricular events are not highly associated,e.g. at least one ventricular ectopic event may be present. As such arelatively large VA interval range, as defined by the selected thresholdvalue, is indicative of SVT.

When the number of atrial sense events equals N, as determined at block514, the degree of 1:1 correspondence between atrial and ventricularevents during the N cycles is analyzed at block 516. If 1:1correspondence between each ventricular sense event and each atrialevent does not exist for all N ventricular cycles, an AAV pattern ispresent indicative of SVT. SVT is detected at block 518. Examples of AAVinterval patterns that correspond to SVT are shown in the insettimelines 432 and 436 shown in FIG. 10.

If each of the N ventricular cycles is associated with one atrial senseevent at block 516, i.e. all N cycles have 1:1 correspondence betweenatrial and ventricular sense events, additional analysis of the timingrelationship between atrial and ventricular sense events is required todiscriminate between SVT and VT. In one embodiment, the AV intervalrange is determined and compared to a threshold at block 520. The AVinterval range may be determined by measuring the AV interval for eachof the N ventricular cycles and identifying the maximum and minimum AVintervals. The AV interval range is the difference between the maximumand minimum. It is recognized that other methods may be used forcomputing a metric of the range or variation of AV intervals during theN ventricular cycles.

If the AV interval range is greater than a threshold, such asapproximately 30 ms, as determined at block 520, VT is detected at block522. If the AV interval range is less than the threshold, SVT isdetected at block 524. Analogous to the previously described analysis ofthe VA interval, if the AV interval is regular, i.e. a small AV intervalrange, the ventricular events are likely being conducted regularly fromthe atria. As such, a relatively small AV interval range as defined bythe selected threshold is evidence of the tachycardia originating in thesupraventricular region and being conducted to the ventricles.

If the AV interval range is large, at least one atrial sense event isunlikely to be associated with a subsequent ventricular sense event,even though they occur with 1:1 correspondence. This suggests that thefast ventricular rate is not originating in the atria. For example, thefirst atrial sense event may be a premature atrial contraction followingpacing and subsequent atrial sense event may be conducted in 1:1correspondence from the ventricles. In the case of a large AV intervalrange, VT is detected at block 522.

Method 500 is described based on identifying N ventricular cycles. It iscontemplated, however, that a similar analysis could be performed byidentifying N atrial cycles and subsequently evaluating the relationshipof ventricular sense events, in timing and in pattern, to the atrialevents.

In summary, instead of identifying only the earliest occurring sensedintrinsic event following the dual chamber pacing, the method shown inflow chart 500 includes analysis of a group of post-pace cardiac cyclesto determine pattern and timing relationships between atrial andventricular sensed events that can be used to reliably discriminate VTand SVT. This analysis generally includes identifying the degree of 1:1correspondence or lack thereof, and identifying the variability of thetime intervals between atrial and ventricular sensed events. While aparticular algorithm is shown in FIG. 11, it is recognized that numerousvariations of methods for analyzing a group of post-pace cardiac cyclesmay be conceived for discriminating the possible patterns and timingrelationships of atrial and ventricular sense events and correlatingthose patterns and timing relationships to either VT or SVT.

Thus, a cardiac medical device and associated method have been presentedin the foregoing description with reference to specific embodiments. Itis appreciated that various modifications to the referenced embodimentsmay be made without departing from the scope of the invention as setforth in the following claims.

1. A cardiac medical device for delivering anti-tachycardia pacing,comprising: a plurality of electrodes implantable within a patient forsensing cardiac signals and delivering cardiac pacing pulses; a therapydelivery module coupled to the plurality of electrodes for deliveringpacing pulses to the patient's heart via the plurality of electrodes;and a controller coupled to the therapy delivery module and adapted tobe electrically connected to the plurality of electrodes, the controllerconfigured to: detect a tachycardia episode from the cardiac signals,schedule one single-chamber pacing pulse in response to detecting thetachycardia episode, control the therapy delivery module to deliver, viathe plurality of electrodes, the one single-chamber pacing pulsefollowed by a plurality of dual chamber pacing pulses, the onesingle-chamber pacing pulse delivered in an atrial chamber at a pacedventricular-atrial interval following a ventricular sense event and afirst one of the plurality of dual chamber pacing pulses delivered at apacing cycle length following the ventricular sense event, the onesingle-chamber pacing pulse delivered at a prematurity interval earlierthan the first one of the plurality of dual chamber pacing pulses sothat the atrial chamber is captured by both the one single-chamberpacing pulse and the first one of the plurality of dual chamber pacingpulses; identify at least one intrinsic event subsequent to theplurality of dual chamber pacing pulses; and classify the detectedtachycardia episode as one of ventricular tachycardia andsupraventricular tachycardia in response to the at least one intrinsicevent.
 2. The device of claim 1 wherein the at least one intrinsic eventcomprises an earliest occurring intrinsic event subsequent to theplurality of dual chamber pacing pulses.
 3. The device of claim 1,wherein the therapy delivery module delivers the scheduled singlechamber pacing pulse in an atrial chamber.
 4. The device of claim 3wherein controller is configured schedule the single chamber pacingpulse during a sensed ventricular-atrial interval.
 5. The device ofclaim 1 wherein the controller is configured to estimate an intervalbetween an expected intrinsic event and a first one of the plurality ofdual chamber pacing pulses; wherein scheduling the single chamber pacingpulse comprises scheduling the single chamber pacing pulse earlier thanthe expected intrinsic event.
 6. The device of claim 1, furthercomprising a memory for storing an estimated physiological refractoryperiod; wherein the controller is configured to schedule the singlechamber pacing pulse at a time interval before the plurality of dualchamber pacing pulses that is greater than the estimated physiologicalrefractory period.
 7. The device of claim 1 further comprising a memorystoring an estimated physiological refractory period; wherein thecontroller is further configured to: control the therapy delivery moduleto deliver the plurality of dual chamber pacing pulses at a pacing cyclelength that is shorter than a cycle length of the detected tachycardiaepisode; measure an interval between a sensed event in a first chamberand a next sensed event in a second chamber during the tachycardiaepisode; compute a time interval between a last cardiac sensed event inthe second chamber and the scheduled single chamber pacing pulse usingthe pacing cycle length; and deliver the single chamber pacing pulse inthe first chamber if a sum of the computed time interval and themeasured interval is greater than the expected physiological refractoryperiod and withhold the single chamber pacing pulse if the sum is lessthan the expected physiological refractory period.
 8. The device ofclaim 1 wherein the controller is further configured to apply atachycardia discrimination blanking interval following a last one of thedual chamber pacing pulses; wherein the least one intrinsic event sensedsubsequent to the plurality of dual chamber pacing pulses comprises anearliest occurring intrinsic event sensed after the blanking interval.9. The device of claim 8 wherein the controller is further configured toset the tachycardia discrimination blanking interval approximately equalto a cycle length of the detected tachycardia episode.
 10. The device ofclaim 1 wherein the controller is further configured to compare amorphology of an intrinsic event sensed subsequent to the dual chamberpacing pulses to a known event morphology for use in classifying thetachycardia episode.
 11. The device of claim 10 wherein the controlleris further configured to identify a 1:1 correspondence of sensed eventsin a first chamber and sensed events in a second chamber and perform themorphology comparison in response to identifying the 1:1 correspondence.12. The device of claim 1 wherein the controller is further configuredto: identify a predetermined number of cardiac cycles in a first chambersubsequent to the dual chamber pacing pulses; determine how manyintrinsic events are sensed in a second chamber during the identifiedcardiac cycles as a number of second chamber events; and classify thetachycardia episode in response to the number of second chamber events.